Allowance for Credit Losses on Reinsurance Recoverables (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($)
Allowance for Credit Losses
Provision for credit losses	$ 3.5	[1]
Balance as of March 31, 2020	6.8
Accounting Standards Update 2016-13
Allowance for Credit Losses
Balance as of January 1, 2020	$ 3.3	[2]

[1] Primarily reflects an increase in the estimate of expected credit losses based on current conditions and reasonable and supportable forecasts as of March 31, 2020 arising from the Pandemic, as defined in Note 9(a) of this Form 10-Q.

[2] See Note 1(c) of this Form 10-Q for further information regarding Alleghany’s adoption of new credit loss accounting guidance.